In my experience, the lighter you go the more blurry 'failure' becomes. Therefore, I'm inclined to say that it's good practice to focus mainly on form, range of motion and timing. When one of those start to fade, it's a good idea to stop.

I'm just saying that whatever 'amount of intensity' I will say, will completely depend on those factors. I can easily perform 15 reps with horrible form without going to failure. Failure in higher rep-ranges is mainly driven by those factors. Therefore, my personal recommendation would be:
- Pick a weight you can perform at least 12 reps with good form.
- If you reach 15+ reps without form breakdown, increase weight next time.
- If form breaks down before hitting 15 reps, keep the weight.